In the following we present the initial function requirements and the product finally developed.

1.1 : Function requirements.

a) Thermal insulation

The insulation concerns two aspects. Firstly the double glazing insulation has to be adapted to the thermal insulation of low energy houses with a positive energy exchange — out of collector function — over the year. Secondly the insulation of the solar collector has to limit the exchange with the inner of the building.

b) Solar collector

The solar collector function has to be optimized to insure part of the heating loads in the middle seasons and to reach a global productivity of 385 kWh/m2y (German criteria for subvention until June 2004).

c) Transparency

The initial target was to have a full visibility of 50% through the glazing. This point is in contradiction with the solar collector function.

d) Variable solar control

As a Venetian blind, dependant on the season and time, the aim is to favor the passive income and to limit the summer over-heating. This point takes into account the sun and daylight dependant on seasons and orientations too.

Over its technical functions, other criteria were also considered :

— Modular dimensions, as usual in conception of facades.

— Attractive design.

— Capacity to respect the normative tests of collectors and glazings.

Global target : The integration of the whole function in the glass collector has to be at least as good as each function in separate products. Further, our goal was to remain in conventional installation and connecting methods.

2.2: Composition.

The actual conception is a compromise between performances of the different functions. The diagram illustrates the composition of the glass collector composed of an insulated glass where a solar collector partially transparent is inserted inside.

Figure 1: Schematic cut of the glass collector The glass collector is composed of :

An insulated glass where the exterior glass, position 1-2, is an extra white glass. On the inside glass, position 3, is a vacuum sputtered low emittance coat. This coating principally limits radiant energy transfers between interior-exterior. Secondly, because of the low emissivity of the collector, this coating limits the transfers between the collector and the interior of the building.

The two glasses are separated by an edge spacer filled with desiccant to regulate humidity inside the double-glazing. The cohesion and gas-tightness is obtained by two glued barriers made of butyl and polyurethane. The interior of the double glazing is full of rare gas.

A solar collector is made of a copper serpentine where aluminum fins are “clipsed”. This fins are coated with a very effective solar-selective deposit with low emissivity and high absorption. A thermoplastic mechanical tightness guarantees the passage of the copper tubes crossing through the edge seal. In the serpentine, water collects and transfers the absorbed energy.

The reflectors are just behind the absorber, vacuum deposited directly on the glass, position 3. They tend to limit, like a Venetian blind, the solar gain. They reflect the intercepted solar radiation to the solar collector. This reflector deposit looks like a mirror on

both sides, with a design function too, because it hides the collector view from the inside as well.

So constituted, solar radiation can be separated in three operative parts figure 1:

• A first part, nearly constant, which directly intercepted by the solar collector.

• A second variable part, which is reflected from the reflectors behind the solar collector.

• A third variable part which is the passive solar gain.

With the sun rising, the amount of solar radiation intercepted from the reflectors increases in profit of the solar collector, until eventually to be total.